1. Field of the Invention
The invention relates to a control apparatus for an internal combustion engine and, more particularly, to an apparatus capable of restraining deterioration of exhaust emissions of a multicylinder internal combustion engine when the air/fuel ratio varies among the cylinders of the engine.
2. Description of Related Art
Generally, in an internal combustion engine equipped with an exhaust emission control system that uses a catalyst, it is essential to control a mixing ratio of air and fuel in a mixture that is burned in the internal combustion engine, that is, the air/fuel ratio, in order to accomplish high-efficient removal of pollutants from exhaust gas through the use of a catalyst. In order to perform such a control of the air/fuel ratio, an air/fuel ratio sensor is provided in an exhaust passageway of the internal combustion engine, and a feedback control is carried out so as to cause the air/fuel ratio detected by the sensor to be equal to a predetermined target air/fuel ratio.
Usually, in a multicylinder internal combustion engine, the air/fuel ratio control is performed by using the same control amount for all the cylinders; therefore, despite the execution of the air/fuel feedback ratio control, the actual air/fuel ratio can vary among the cylinders. In such a case, if the variation of the air/fuel ratio is of a small degree, the variation of the air/fuel ratio can be absorbed by the air/fuel ratio feedback control and pollutants in exhaust gas can be removed by the catalysts. Thus, small degrees of variation of the air/fuel ratio do not affect the exhaust emissions.
However, if the air/fuel ratio greatly varies among the cylinders due to, for example, failure of the fuel injection system of at least one cylinder or the like, exhaust emissions deteriorate.
It is desirable that such a large variation of the air/fuel ratio be detected as an abnormality. Particularly, in the case of the internal combustion engines used in motor vehicles, in order to prevent a vehicle from traveling with deteriorated exhaust emissions, it has been demanded that an inter-cylinder air/fuel ratio variation abnormality of the engine be detected in a vehicle-mounted state (an on-board state). For example, in an apparatus described in Japanese Patent Application Publication No. 2009-30455 (JP 2009-30455 A), the inter-cylinder air/fuel ratio variation abnormality is detected on the basis of divergences of the outputs of air/fuel ratio sensors disposed upstream and downstream of a catalyst.
Typically, the foregoing air/fuel ratio feedback control is performed on the basis of the output of the air/fuel ratio sensor disposed upstream of the catalyst, that is, the pre-catalyst sensor. Furthermore, if the air/fuel ratio of a cylinder greatly deviates to the rich side in a multicylinder internal combustion engine, the output of the pre-catalyst sensor deviates to the rich side from a true air/fuel ratio due to the influence of hydrogen discharged from the cylinder of rich deviation (see JP 2009-30455 A).
If, in such a state, the air/fuel ratio feedback control is performed in a usual manner, the actual air/fuel ratio of exhaust gas deviates to the lean side of the target air/fuel ratio, giving rise to possibility of increase in the amount of NOx emissions.
A conceivable countermeasure against this is to restrain or offset a lean deviation of the actual exhaust air/fuel ratio by enriching the air/fuel ratio if an inter-cylinder air/fuel ratio variation is detected.
However, according to results of researches made by the present inventors, it has turned out that if such enrichment of the air/fuel ratio is continued, the enrichment can become excessive and result in increased amounts of HC emissions.